linux-kernel - Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks

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Message-ID: <ebf5d7357f194debc3bd6e91e0a8da4202c87653.camel@linux.ibm.com>
Date:   Wed, 09 Oct 2019 15:09:41 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Song Liu <songliubraving@...com>, Michal Hocko <mhocko@...e.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Keith Busch <keith.busch@...el.com>, linux-mm@...ck.org,
        Paul Mackerras <paulus@...ba.org>,
        Christoph Lameter <cl@...ux.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        linux-arch@...r.kernel.org, Santosh Sivaraj <santosh@...six.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        kvm-ppc@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Logan Gunthorpe <logang@...tatee.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Roman Gushchin <guro@...com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to
 monitor lockless pgtable walks

On Fri, 2019-10-04 at 13:28 +0200, Peter Zijlstra wrote:
> > Could you please explain it?
> > I mean, why this breaks tmpfs-thp?
> > Also, why mm_cpumask() is also broken?
> 
> Because shared pages are not bound by a mm; or does it not share the thp
> state between mappings?

By what I could understand, even though the memory is shared, the
mapping may differ for different processes (i.e. the same physical
memory that is mapped as a hugepage in process A can be mapped as a lot
of smallpages in process B).

Did I miss something here?  

> And once you (re)figure it out, please write it down. It is a crucial
> bit of the puzzle and needs to be part of the Changelogs.

I am still investing time studying this. More on this later :)

Thanks!

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